In the last two decades, radio communications technologies have developed enormously and the radio communications technologies are emerging continuously. Radio communications networks increasingly occupy people's lives by virtue of its enormous flexibility, and become an indispensable part of people's lives.
However, radio spectrum resources are limited. With a sharp increase of persons who use the radio communications networks and with increasingly higher requirements on performance of the radio communications networks, a shortage of spectrum resources has become a key limitation on radio communication performance. Currently, a cellular network is a prevailing radio communications network. In this communication network, communication between two terminals needs to be forwarded by a base station, and a same packet, which is transmitted from a terminal to a base station and then from the base station to the terminal, occupies an air interface resource twice. If the two communications terminals are in a relatively long distance and incapable of reaching each other, this solution is relatively feasible. However, if two sides of communication are relatively close to each other within a communication scope of the other side, transmitting a packet between the terminals directly without the need to be forwarded by a base station can save half of resources.
Direct communication between a device and a device (D2D communication for short, Device to Device) enables direct communication between terminal devices without requiring any intermediate infrastructure. Therefore, the direct communication between terminal devices can use spectrum resources more efficiently, increase a capacity of a cellular network, and reduce overheads of control signaling of a base station, and is a technology that can bring enormous benefits to cellular network communications. Because the D2D communication is direct communication between terminals, a paging message needs to be directly sent from a paging terminal to a paged terminal without requiring assistance of the base station or a core network. Some conventional technologies can implement D2D communication, such as wifi, BT, and ad hoc. However, these systems all work in asynchronous mode. Therefore, in a D2D communications system in the prior art, system devices all work in asynchronous mode, and as a result a UE cannot effectively discover other UEs in a process of mutual discovery of D2D user equipments (User Equipment, UE), wasting system power.